There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
CMOS imager devices are well known. CMOS imager devices are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Imager devices,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366 and 6,326,652; assigned to Aptina Imaging Corporation, the entire disclosures of which are incorporated herein by reference.
CMOS imager devices typically include an array of pixels, which converts light energy received, through an optical lens, into electrical signals. Each pixel contains a photoconversion device for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photoconversion devices are processed to render a digital image.
The amount of charge generated by the photoconversion device corresponds to the intensity of light impinging on the photoconversion device. Accordingly, it is important that all of the light directed to the photoconversion device impinges on the photoconversion device rather than being reflected or refracted toward another photoconversion device as optical cross-talk.
For example, optical cross-talk may exist between neighboring photoconversion devices in a pixel array. In an ideal imager, a light enters only through the surface of the photoconversion device that directly receives the light stimulus. In reality, however, some light intended for one photoconversion device also impinges on another photoconversion device through the sides of the optical path existing between a lens and photoconversion device.
Optical cross-talk can bring about undesirable results in the images produced by the imaging device. The undesirable results can become more pronounced as the density of a pixel in imager arrays increases, and as pixel size correspondingly decreases. The shrinking pixel sizes make it increasingly difficult to properly focus incoming light on the photoconversion device of each pixel without accompanying optical cross-talk.
Optical cross-talk can cause a blurring or reduction in contrast in images produced by the imaging device. Optical cross-talk also degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel and device sizes are reduced. Furthermore, degradation caused by optical cross-talk is more conspicuous at longer wavelengths of light. Light having longer wavelengths penetrates more deeply into the silicon structure of a pixel, providing more opportunities for the light to be reflected or refracted away from its intended photoconversion device target.
Accordingly, there is a need and desire for an improved apparatus and method for reducing optical cross-talk in imaging devices.